1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser and an image forming apparatus using the vertical cavity surface emitting laser.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) can extract light in a vertical direction relative to a semiconductor substrate. Therefore, a two-dimensional array can be easily formed with VCSELs.
Various industrial applications of a surface-emitting laser array are expected, and for example, if a high-density two-dimensional laser array is used, applications to a high-speed high-definition electrophotographic system are possible. In an electrophotographic process, since stable and minute laser spots need to be formed on a photoconductor drum, a stable operation of a single transverse mode and a single longitudinal mode is necessary as a laser characteristic. By using the surface-emitting laser, a current confinement layer can be provided by selective oxidation in an element to limit a light emission area of an active layer, which realizes single transverse mode formation. However, if single transverse mode oscillation is attempted based only on this method, the oxide confinement diameter needs to be reduced to about 3 μm, which reduced oxide confinement diameter derives a reduced light emission area and thus it is difficult to obtain a large laser output.
Japanese Patent Application Laid-Open No. 2009-147302 proposes a surface-emitting laser as a method for single transverse mode formation under a larger oxide confinement diameter, the surface-emitting laser attains a single transverse mode oscillation at a high output by elongating a resonator to enlarge a diffraction loss of a higher-order transverse mode compared to a diffraction loss of a basic transverse mode.
The optical layer thickness of a resonator often used in a general surface-emitting laser is about one wavelength or two wavelengths. For example, the physical length of resonator in one-wavelength resonator is conventionally about 0.3 μm in a laser of a 980 nm band used in H. J. Unold et al., IEEE Photonics Technology Letters, Vol. 12, No. 8, August 2000, p. 939-941. Meanwhile, in Unold et al., a spacer layer of 2 to 8 μm is provided in the resonator to increase the resonator length to attain the single transverse mode oscillation with a current confinement layer having an oxide confinement diameter of 7 μm.
S. W. Corzine et al., Photo. Tech. Lett., Vol. 1, No. 3, 1989, p. 52-54 illustrates reducing a threshold current and improving the efficiency by positioning an active layer at a standing wave loop to increase the gain of the active layer.